1. Field of the Invention
The present invention relates to a photoelectric conversion element. More particularly, the present invention is concerned with a photoelectric conversion element comprising a composite dye and an n-type semiconductor, the composite dye comprising a plurality of component dyes which have different excitation levels and which are chemically bonded to each other to form a straight chain or branched structure for transferring an electron therethrough, wherein the straight chain or branched structure is, at one end thereof, secured to the n-type semiconductor and has, at least at one other end thereof, a free end, and wherein the plurality of component dyes are arranged in an order such that the excitation levels of the plurality of component dyes are decreased as viewed from the one end of the structure toward the at least one other end of the structure. The photoelectric conversion element of the present invention exhibits excellent photoelectric conversion properties, especially high efficiency in converting solar energy to electric energy (i.e., high energy conversion efficiency), and a dye-sensitized solar battery can be easily produced therefrom. Therefore, the photoelectric conversion element of the present invention can be advantageously used for a dye-sensitized solar battery and the like.
The present invention is also concerned with a dye-sensitized solar battery using the photoelectric conversion element.
2. Prior Art
Consumption of energy is indispensable to civilized society. Most of the energy which is consumed by civilized society is derived from fossil fuels, in which sunray energy has been accumulated over many years. In recent years, the problem that the amount of fossil fuels available is being reduced and the problem that the burning of fossil fuels causes global warming have arisen, and there is an increasing fear that these problems will be obstacles to the sustainable development of human society.
For solving the above-mentioned problems, various studies have been made to directly utilize sunray energy. Among these studies, the studies on solar batteries have been vigorously made, because solar batteries exhibit high efficiency in converting solar energy to electric energy (i.e., high energy conversion efficiency). Among the solar batteries, special attention has been paid to a dye-sensitized solar battery, which uses a photosensitizer, such as a dye, and which is capable of efficiently taking out electrons from the photosensitizer by the irradiation of the photosensitizer with sunray. Specifically, since Michael Gratzel et al. reported a system which uses a dye-sensitized solar battery having an energy conversion efficiency of more than 7% (see Nature 1991, 353, 737), a dye-sensitized solar battery has drawn special attention as the next-generation solar battery which can be produced at a low cost without use of a complicated method.
In general, a dye-sensitized solar battery has the following structure. A substrate (electroconductive support) is laminated on a support made of glass, a polymer or the like, wherein the substrate has coated thereon an indium oxide membrane (e.g., an ITO (indium tin oxide) membrane) or fluorine-doped tin oxide (FTO) membrane having excellent conductivity and excellent transparency. On the substrate is laminated a porous titanium oxide membrane, as an n-type semiconductor, which is a cheap material and has a size of several tens of nanometers, thereby forming a laminate as a photo-anode. A thin layer of platinum is laminated on a substrate which is substantially the same as the above-mentioned substrate, thereby forming a laminate as a cathode. Between the photo-anode and the cathode is interposed an electrolytic solution containing a redox couple, such as iodine, thereby forming a structure in which a photo-anode and a cathode face each other through an electrolytic solution. In this structure, the photo-anode has carried thereon a dye (such as a complex dye) as a photosensitizer for the purpose of absorbing visible light from sunray to thereby generate excited electrons from the photosensitizer, so that the photo-anode functions as a photoelectric conversion element.
The excited electrons generated from the photosensitizer are transferred to the n-type semiconductor, and further transferred to the cathode through a conductor which connects the photo-anode and cathode. The excited electrons having been transferred to the cathode reduce the electrolytic solution and, in turn, the electrolytic solution reduces the photosensitizer having been oxidized by the emission of electrons from the photosensitizer. By repeating the above-mentioned series of operations, the dye-sensitized solar battery works.
A photosensitizer, such as a dye, can absorb light having wavelengths within a certain range. When the photosensitizer is irradiated with such light having wavelengths within the range, the photosensitizer receives the energy of photons. As a result, electrons in the ground state, contained in the photosensitizer, are excited and transferred to excited states. In general, excited electrons emit energy in the form of heat or light, such as fluorescence or phosphorescence, and return to the ground state. However, when electrons in the photosensitizer are excited, conversion of light energy to electric energy (i.e., photoelectric conversion) is done by taking out the excited electrons from the photosensitizer.
As seen from the above, a photosensitizer plays an important part in the conversion of light energy to electric energy. Therefore, studies on photosensitizers have been vigorously made.
When the function of a photosensitizer is discussed in terms of molecules, the photosensitizer generally receives one photon to excite one electron contained therein. The longer the wavelength of a light, the smaller the energy of the light. It follows from this that a photosensitizer which absorbs a long-wavelength light to excite electrons contained therein (i.e., make electrons contained therein transferred to excited states) can excite electrons contained therein by absorbing a low energy light. Therefore, the photosensitizer can absorb a wide range of light, which ranges from a long-wavelength light to a short-wavelength light, which has a large energy. In the application of a solar battery, for taking out a number of electrons (i.e., obtaining a high electric current), it is important to effectively utilize a wide range of light contained in sunray, which has a wide distribution of light wavelengths. In view of this, various attempts have been made to develop a photosensitizer capable of absorbing longer wavelength light from sunray.
For this purpose, i.e., for obtaining a photosensitizer capable of absorbing longer wavelength light from sunray, it is generally attempted to enlarge a conjugate structure. For example, Japanese Patent Application Prior-to-Examination Publication (Tokuhyo) No. 2002-512729 (corresponding to WO98/50393 and U.S. Pat. No. 6,245,988) discloses a technique in which a single nuclear complex dye having a tridentate ligand is used. On the other hand, Inorg. Chem. 2002, 41, 367 discloses a technique in which a single nuclear complex dye having a quadridentate ligand is used. Further, J. Phys. Chem. B 2003, 107, 597 discloses a technique in which an organic dye having a conjugate structure is used.
Moreover, for obtaining a photosensitizer capable of absorbing longer wavelength light from sunray, a technique using a multinuclear complex having a plurality of metals is disclosed in Unexamined Japanese Patent Application Laid-Open Specification No. 2000-323191 (corresponding to EP1052661). Also, for using a plurality of dyes in combination, a technique in which a plurality of dye layers are laminated is disclosed in Unexamined Japanese Patent Application Laid-Open Specification No. 2000-195569, and a technique in which a plurality of dyes are associated with each other is disclosed in Unexamined Japanese Patent Application Laid-Open Specification No. 2002-343455.
However, the above-mentioned techniques, i.e., the techniques in which a single dye is used for absorbing longer wavelength light from sunray, and the techniques in which a plurality of dyes are used in combination (wherein the dyes receive excited electrons having the same energy level from the electrolyte and transfer the excited electrons to the n-type semiconductor), have a theoretical limit on the energy conversion efficiency when electrons are taken out from sunray having a wide distribution of wavelengths. The reason for this is as follows. As a dye absorbs light having longer wavelengths from sunray, the number of electrons taken out from the dye is increased, so that a larger electric current can be obtained. However, light having a long wavelength has a small energy, so that the energy which can be used for transferring an electron to an excited state is inevitably small. Therefore, a high voltage cannot be obtained.
As mentioned above, in the case of a general photosensitizer, the energy of only one photon is used for exciting a single electron (one-photon absorption). However, in the case of a photosensitizer comprising a specific compound, the energies of two photons can be used for exciting a single electron (two-photon absorption) (see Science 1998, 281, 1653). In such case, it becomes possible to transfer an electron to a high energy level by using only low-energy light having long wavelengths and, hence, the above-mentioned theoretical limit can be overcome. The technique of the two-photon absorption is a technique in which an electron having been excited in a molecule is further excited in the molecule. An electron in an excited state returns to the ground state in a short period of time. Therefore, in the technique of the two-photon absorption, generally, the electron is transferred to a quasi-stable excited state (such as a triplet state) so that the life time of the excited electron (i.e., excitation lifetime) becomes longer, thereby assuring a time sufficient for causing a second excitation of the electron. However, even in the technique of the two-photon absorption, it is necessary for a single molecule to absorb light twice in a short period of time and, hence, the probability that the second excitation of the electron occurs is small, so that it is difficult to take out a number of electrons from the photosensitizer. Therefore, it is difficult to apply the two-photon absorption technique to fields (such as the field of solar batteries) in which it is necessary to take out a number of electrons. Thus, the fields to which it is attempted to apply the two-photon absorption technique are limited to the field of polymerization initiators (see Nature 1999, 398, 51) and the field of photosensors (see Unexamined Japanese Patent Application Laid-Open Specification No. 2001-210857).
In addition, as a technique for efficiently taking out solar energy, J. He et al. propose a technique in which two electrodes facing each other are respectively provided with n-type and p-type semiconductor layers, wherein the semiconductor layers are respectively sensitized by dyes having different excitation levels (see Solar Energy Materials & Solar Cells 2000, 62, 265). However, this technique has a problem in that each of the provision of a semiconductor layer and the adsorption of a dye must be performed a plurality of times, thereby rendering combersome the production of a system used in the technique.